1. Field of the Invention
The present invention relates generally to local area networks for interconnecting computers and, in particular, to concentrators for token ring local area networks having regeneration and retiming circuitry.
2. Description of Related Art
Local area networks are commonly used for interconnecting computers. One type of network is the well known token passing ring ("token ring") network which typically includes a star-wired ring utilizing a structured cabling system. A set of stations, such as personal computers, are serially connected by a transmission media.
FIG. 1 depicts a representative token ring network which includes three concentrators 10, each of which is typically located in a separate wiring closet. The concentrators 10 are connected together by way of trunk cables 12. Each concentrator 10 has five ports 11, three of which may be connected to separate data terminal equipment (DTE) 14, such as personal computers. The remaining two ports 11 are for connecting to other concentrators. The DTE 14 are connected by way of lobe cables 16 which extend from the DTE to the wiring closet in which the associated concentrator 10 is located and then to the concentrator itself.
The network includes two functional types of stations (DTEs), including one station which acts as an active monitor and the remaining stations which function as standby monitor stations. The active monitor has a number of important functions. One function is to send out the tokens which circulate around the ring for other stations to append to their data packets. All data packets make one complete transit around the ring from the source station to the destination station and back to the source station. The active monitor also sets the frequency of the ring by sourcing its transmission from an internal crystal oscillator. Furthermore, the active monitor absorbs the jitter which is accumulated as the data transverse the ring.
Each station or DTE includes an adapter card for connecting the station to the network. FIG. 2 is a very simplified block diagram of the clocking elements of the adapter card. Differential Manchester encoded data from the previous station is received on line 18 and fed to a phase locked loop (PLL) 20 which extracts a clock from the data. In the case of the active monitor, the extracted clock is used to load the data into a latency buffer which includes an elastic buffer 22. A clock generated by a crystal oscillator 24 is then used to read the data out of the buffer for forwarding to the next station of the ring, as shown by line 26. In the case of the standby monitor stations, the extracted clock from the PLL 20 is used for both receiving and transmitting the data.
The jitter accumulated by the data as it traverses the ring can be grouped into two categories. The first is systemic jitter caused by intersymbol interference, cable dispersion and non-ideal circuit elements within the ring path. Systemic jitter is proportional to the number of stations on the ring. The second type of jitter is random jitter which is generally attributable to noise sources such as non-ideal circuit elements and crosstalk.
The total accumulated jitter on the ring must be less than the size of the elastic buffer 22 in order for the ring to operate. Thus, the buffer size determines the maximum number of stations on the ring. Another limiting characteristic in the token ring is the eye closure. Cable attenuation and induced jitter are the two dominant factors affecting eye opening. This serves to define the maximum lobe cable 16 (FIG. 1) length, maximum number of concentrators and the maximum trunk cable 12 (FIG. 1) length.
The frequency of operation of token rings has increased from 4 Mb/s to 16 Mb/s which magnifies the problems associated with jitter. Furthermore, the use of unshielded twisted pair (UTP) has become more popular which results in greater crosstalk in comparison to shielded twisted pair (STP). This also causes increased jitter.
There are passive concentrators which simply route the received data from one DTE to the next DTE on the ring. Such concentrators do nothing to reduce the effects of jitter. Active concentrators have been used which regenerate the data, but do not provide retiming. These concentrators function to minimize jitter associated with crosstalk, but do not filter the high frequency jitter as does the phase locked loop in each station. There has been a suggestion to provide both a regenerating function and a retiming function in concentrators. The regeneration function restores the signal amplitude and the retiming function, to be carried out utilizing a PLL similar to that used in the stations (FIG. 2), removes some of the unwanted jitter.
While the use of a PLL similar to that in the stations is an acceptable method of recovering a clock, the increased complexity of the circuitry causes severe penalties to implementation in the concentrator. These disadvantages include increased cost, lower reliability, and the need for a significant amount of area on the printed circuit board in the concentrator which ultimately limits the number of ports which may be implemented on the concentrator.
Integrated circuit PLLs are less than satisfactory for the present application because they typically use a low performance R-C multivibrator which produces a high phase noise. A very wide band loop filter is required to remove the wide band phase noise associated with such multivibrators. For example, for operation at 16 MB/s, the loop bandwidth is 1 MHz. This requires active components in the loop filter with bandwidths of at least 10 MHz to avoid instability in the PLL due to inadequate phase margins. This is not easily achieved.
The present invention overcomes the above-noted problems which arise when an active concentrator utilizes a PLL for clock recovery. The disclosed concentrator provides timing recovery without the attendant problems of PLLs. The circuitry is reduced because the requirement for a separate voltage controlled oscillator, phase comparator, loop filter and frequency acquisition aid is eliminated. These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following Detailed Description of the Invention together with the drawings.